Dealing with filters and colour's

Question

I want to make filters like shown here

• these are my target filters but can you please guide me how to go for them
• how i can make filters like these?
• which algorithms i need to follow? and which step i need to take as beginner?
• Which is the better and easiest way to get the values of RGB and shades of filters .

copy of image from link above by spektre:

• the source image is the first after camera in the first line.

Answer 1

very hard to say from single non test-screen image.

1. the black and white filter

   is easy just convert RGB to intensity i and then instead RGB write iii color. The simplest not precise conversion is

   i=(R+G+B)/3
   

   but better way is use of weights

   i=w0*R+w1*G+w2*B
   

   where w0+w1+w2=1 the values can be found by a little google search effort

2. the rest

   some filters seem like over exponated colors or weighted colors like this:

   r=w0*r; if (r>255) r=255;
   g=w1*g; if (g>255) g=255;
   b=w2*b; if (b>255) b=255;
   

   write an app with 3 scrollbars for w0,w1,w2 in range <0-10> and redraw image with above formula. After little experimenting you should find w0,w1,w2 for most of the filters ... The rest can be mix of colors like this:

   r=w00*r+w01*g+w02*b; if (r>255) r=255;
   g=w10*r+w11*g+w12*b; if (g>255) g=255;
   b=w20*r+w21*g+w22*b; if (b>255) b=255;
   

   or:

   i=(r+g+b)/3
   r=w0*r+w3*i; if (r>255) r=255;
   g=w1*g+w3*i; if (g>255) g=255;
   b=w2*b+w3*i; if (b>255) b=255;
   

btw if you want the closest similarity you can:

1. find test colors in input image

   like R shades, G shades , B shades , RG,RB,BG,RGB shades from 0-255. Then get colors from filtered image at the same position and draw depedency graphs for each shade draw R,G,B intensities.

   One axis is input image color intensity and the other one is R,G,B intensity of filtered color. Then you should see which formula is used directly and can also compute the weights from it. This is how over-exponation works for Red color

   

2. if the lines are not lines but curves

   then some kind of gamma correction is used so formulas use polynomial of higher order (power of 2,3,4...) mostly power of 2 suffice. In that case the weights can be also negative !!!

3. some filters could use different color spaces

   for example transform RGB to HSV shift hue and convert back to RGB. That will shift colors a little.